The present invention generally relates to packet communications between a base station and a mobile terminal, and more particularly, to packet communications using a modulation and coding set determined based on the channel quality and the amount of data buffered at the sender.
The 3rd Generation Partnership Project (3GPP) of the International Mobile Telecommunication 2000 (IMT-2000) has standardized the specification of High Speed Downlink Packet Access (HSDPA), which is a packet-based data service for W-CDMA (Wideband Code Division Multiple Access) downlink. See 3rd Generation Partnership Project, “Technical Specification Group Radio Access Network; Physical Layer Aspects of UTRA Hight Speed Downlink Packet Access (Release 2000), 3G TR. 25.848, VO. 2.0 (2000-05), which was also accessed over the Internet at URL http://www.3gpp.org on Dec. 12, 2002.
In HSDPA, link adaptation is employed to select a modulation scheme in accordance with the channel quality. To be precise, a high-rate modulation and coding set is used for a mobile station with good channel quality, and the transmission block size (TBS) per unit time is set large. On the other hand, for a mobile station with bad channel quality, a low-rate modulation and coding set is used, and the TBS is set small.
There are various types of traffic patterns arising at a mobile station, and therefore, the amount of data buffered in the transmission buffer of the mobile station for the subsequent transmission is variable. To this end, when the amount of data delayed in the buffer from being transmitted is smaller than the TBS, the mobile station adds padding to the data to be transmitted from the buffer when creating a packet. The padding corresponds to the difference between the TBS and the amount of data to be transmitted from the buffer.
However, since padding does not bear information, it is desired, from the standpoint of efficient use of the radio resources, that the padding field be as small as possible. Meanwhile, since a high-rate modulation and coding scheme is inferior, in protection against noise interference, to a low-rate modulation and coding scheme, data are less likely to be received successfully on the receiving side when a high-rate modulation and coding set is used. For this reason, it is desirable to employ a low-rate modulation and coding set in the case where the amount of data, except for padding, to be transmitted is the same as can be transmitted in a transmission time interval (e.g., a time slot).
An example is given wherein there is a mobile station existing that has good channel quality and holds a small amount of data to be transmitted in the buffer. In this case, the mobile station generally employs a high-rate modulation and coding set because of the good channel quality. However, because the transmission block size (TBS) becomes large when using a high-rate modulation and coding set, a large amount of padding has to be added to the small amount of data to be transmitted, and consequently, the radio resources are wasted. In addition, QoS (Quality of Service) is degraded due to the increased probability of failing at data receiving.